This invention pertains generally to the field of materials which closely mimic the ultrasonic propagation characteristics of human tissue, and particularly to such materials used in ultrasound phantoms for use with ultrasound scanners.
Materials which closely mimic the ultrasonic propagation characteristics of human tissue are employed in imaging phantoms and other test objects for use with ultrasound scanners. These phantoms may be used to carry out performance checks on ultrasound scanners. Phantoms may also be used for training or testing student technologists in the operation of ultrasound scanners or the interpretation of ultrasound images produced by such scanners.
Ideally, such material should be capable of mimicking soft human tissue with respect to at least three characteristics: speed of sound, ultrasonic attenuation, and ultrasonic scattering. The speed of sound in the tissue mimicking material should rest in the range from approximately 1460 m/s, characteristic of human fat tissue, to 1640 m/s, characteristic of the human eye""s lens. The attenuation coefficient with respect to frequency of the material should lie in the range from approximately 0.4 dB/cm/MHz, characteristic of human fat tissue, to 2.0 dB/cm/MHz, characteristic of human muscle tissue. Additionally, the attenuation coefficient should be approximately proportional to the ultrasonic frequency. In other words, the attenuation coefficient with respect to frequency, or the attenuation coefficient slope, should remain constant for varying ultrasonic frequencies. These characteristics of human tissue should be maintained at all frequencies in the typical range of ultrasonic scanners, from 1-10 MHz. Moreover, the variation of these characteristics within the range of room temperature should be small. Additionally, these materials should be stable in time and invulnerable to reasonable environmental fluctuations. They should also be free of any pockets of air or gas. Furthermore, the bulk properties of the material should be the same throughout the volume of a particular phantom or phantom section.
A tissue mimicking material satisfying the above characteristics was disclosed in U.S. Pat. No. 4,277,367, to Madsen, et al., entitled Phantom Material and Methods, in which both the speed of sound and the ultrasonic attenuation properties could be simultaneously controlled in a mimicking material based on water based gels, such as those derived from animal hides. In one embodiment, ultrasound phantoms embodying the desired features for mimicking soft tissue were prepared from a mixture of gelatin, water, n-propanol and graphite powder, with a preservative. In another embodiment, an oil and gelatin mixture formed the basis of the tissue mimicking material.
Tissue mimicking material is typically used to form the body of an ultrasound scanner phantom. This is accomplished by enclosing the material in a container which is closed by an ultrasound transmitting window cover. The tissue mimicking material is admitted to the container in such a way as to exclude air bubbles from forming in the container. In addition to the tissue mimicking material itself, scattering particles, spaced sufficiently close to each other that an ultrasound scanner is incapable of resolving individual scattering particles, and testing spheres or other targets may be located within the phantom container, suspended in the tissue mimicking material body. Such an ultrasound phantom is useful in evaluating the ability of ultrasound medical diagnostic scanners to resolve target objects of selected sizes located throughout the tissue mimicking material. The objective is for the ultrasound scanner to resolve the testing spheres or other targets from the background material and scattering particles. This type of ultrasound phantom is described in U.S. Pat. No. 4,843,866, to Madsen, et al., entitled Ultrasound Phantom.
U.S. Pat. Nos. 5,625,137 and 5,902,748 to Madsen, et al. disclose a tissue mimicking material with very low acoustic backscatter coefficient that may be in liquid or solid form. A component in both the liquid and solid forms is a filtered aqueous mixture of large organic water soluble molecules and an emulsion of fatty acid esters, which may be based on a combination of milk and water. Hydroxy compounds, such as n-propanol, can be used to control the ultrasonic speed of propagation through the material and a preservative from bacterial invasion can also be included. The use of scattering particles allows a very broad range of relative backscatter levels to be achieved.
In an effort to limit patient exposure to ultrasound, the Food and Drug Administration (FDA) and the American Institute of Ultrasound in Medicine have made recommendations to ultrasound equipment manufacturers that values of two parameters, which are relevant to potential biological effects, be available to clinical users. One parameter is the thermal index (TI), the value of which is a predictor for temperature rise of tissue in an ultrasound beam. The other recognized potential mechanism of biological damage involves cavitation, and the likelihood of patient injury due to cavitation is thought to increase with the value of the parameter referred to as the mechanical index (MI). The definitions and models for the TI and MI are detailed in a standard produced by a joint committee of the American Institute of Ultrasound in Medicine (AIUM) and the National Electrical Manufacturers Association (NEMA).
In acoustic output quantification, a hydrophone is typically used in a water tank to record the temporal acoustic pressure of a propagating wave. The most extreme rarefactional pressure, pr, is used for calculating MI, and the pulse intensity integral, PII, is used for calculating the temporal average intensity. The MI and the temporal average intensity are subject to restrictions imposed by the FDA. The temporal average intensity is also used in calculating TI.
According to the presently applicable standards, the water-measured pressures and intensities are made to model the attenuation effects of tissue through the application of a derating factor. Derating is applied to any waveform by multiplying all acoustic pressures, p(t) (at time t) by a factor exe2x88x92xcex1f 0z where xcex1xe2x89xa10.3 dB/cm-MHz=0.0345 nepers/cm-MHz is the attenuation coefficient slope, f0 is the center frequency of the pulse, and z is the distance from the transducer to the receiver. Use of the deration process assumes that the propagation of the sound pulse in water or in tissue is linear. However, it is well known that most diagnostic ultrasound systems commonly emit pulses which are nonlinear for propagation in both water and tissue. Generation of pulse nonlinearities in the water is a concern because the derating process assumes linear propagation and a result can be significant underestimation of intensity. It is not known to what extent such nonlinearities occur in tissue although it is thought that most tissues are less subject to such an effect than water.
There are two approaches to replacing the presently used water-deration method of measurement with more acceptable methods. One is theoretical, viz, to extend the deration method to account for nonlinear propagation in both water and a hypothetical tissue-mimicking material using the Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation. The other approach for accurately determining exposure parameters is direct experimental measurement using a medium which adequately mimics tissue in the relevant ultrasonic parameters, viz, propagation speed, attenuation and nonlinear propagation. The liquid tissue mimicking material disclosed in U.S. Pat. Nos. 5,625,137 and 5,902,748 may be utilized for this purpose. Tests of the accuracy of the water-derating procedure have been made employing a tissue-mimicking liquid as described in U.S. Pat. Nos. 5,625,137 and 5,902,748 with properties recommended by the American Institute of Ultrasound in Medicine (AIUM) and having a B/A value typical of soft tissue. The peak PII found in the tissue-mimicking liquid is more than twice that measured under identical conditions in water (and derated). Another example is rarefactional pressures measured in such tissue mimicking liquid and in water (with derating), where the peak value in the tissue mimicking liquid is found to be about twice the peak water-derated value. However, there are two disadvantages in the use of this liquid material and the open tank measurement method. Firstly, material made with condensed whole milk has the long term (a few months) disadvantage that lipid particles collect at upper surfaces, thus compromising the spatial uniformity of ultrasonic properties. Secondly, if this liquid is left for long periods (a few months) exposed to air, desiccation and clumping occur. If condensed whole milk containing either thimerosal or 1-(cis-3-chlorallyl)-3,5,7-triaza-1-azonia adamantane chloride as a preservative is stored in a container so that it is isolated from air, the clumping mentioned does not occur. Sedimentation of the lipids cannot be avoided, however, and to assure uniformity of properties after storage of the tissue mimicking liquid for weeks or months, the liquid must be vigorously agitated before the tests can be performed.
In accordance with the invention, a tissue mimicking material for ultrasound phantoms is provided that has a speed of sound and ultrasound attenuation that are characteristic of human tissue, and that is well suited to be used for measuring and calibrating the potential biological effects of ultrasound equipment. The material provides more realistic tests of the biological effects of ultrasound equipment than conventional water based tests. For use in testing biological effects of ultrasound equipment, the tissue mimicking material of the invention is provided in a liquid form having preferred characteristics at normal room temperature of a suitable propagation speed characteristic of human tissue and a constant attenuation coefficient slope. The liquid tissue mimicking material is stable in its characteristics over extended periods of time, allowing the material to be utilized repeatedly over long periods of time to provide comparative tests of ultrasound equipment, including periodic tests of a particular medical ultrasound unit to determine changes in the unit""s characteristics over time.
The liquid tissue mimicking material of the invention allows ultrasound scanner beam properties to be determined at arbitrary positions by immersing movable detectors or reflectors at various positions in the container of the phantom containing the liquid tissue mimicking material. Further, phantom test objects may be refilled with materials mimicking ultrasonically different tissues.
The liquid tissue mimicking material of the invention is formed of an ultra-filtered aqueous mixture of large organic water soluble molecules in water that is condensed from milk. A preservative is generally included in the mixture to inhibit bacterial invasion. The material is formed of ultra-filtered skim milk in which the fat content has been reduced below 1% while total solids are in the range of 10% to 30%. For use as a material for testing the biological effects of ultrasound equipment, the tissue mimicking material is preferably concentrated such that the speed of sound in the material at 22xc2x0 C. (typical room temperature) is in the range of 1460 m/s to 1640 m/s, preferably at approximately 1540 m/s, with an attenuation coefficient slope of about 0.3 dB/cm/MHz. The total fat content of the ultra-filtered milk forming the material is preferably less than 0.6%, with total solids preferably about 14% by weight of the material. The total fat content is selected to be sufficiently low and the size of the remaining lipid particles sufficiently small that essentially no separation of lipids from the remaining water and other solid materials occurs over substantial periods of time, many months to a year or more, so that the functional ultrasound characteristics of the tissue mimicking material do not change over time. The maximum diameter of the lipid particles remaining after ultrafiltration is sufficiently small so that the lipid particles do not agglomerate, and the lipid particles remain in suspension over extended periods of time. Preferably, the average lipid particle diameter is less than about 0.02 to 0.03 micrometers. It is believed that by maintaining the total lipids content below about 1%, and by maintaining the remaining lipid particles below a size of 0.02 to 0.03 micrometers in diameter, the lipid particles remain in suspension and do not agglomerate into larger particles which can float by gravity to the surface of the tissue mimicking material over long enough periods of time.
The tissue mimicking material of the present invention may also be embodied in a form suitable for use as a phantom for carrying out performance evaluations on ultrasound scanners. In such phantoms, the tissue mimicking material of the invention may also be in solid form in which a pure gel forming material is included to form an elastic solid tissue mimicking material. Solid scatterers and/or test objects may be added to the solid type of tissue mimicking material. For use in testing the general performance characteristics of ultrasound scanners, the solid tissue mimicking material is preferably concentrated such that the propagation speed is maintained at about 1540 m/sxc2x15 m/s with an attenuation coefficient slope of about 0.5 dB/cm/MHz.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.